FIG. 1 is a schematic illustration of the basic method of generating frequency combs such that the absolute frequency of each comb line is known. The method uses a laser emitting at many modes equally spaced, with a spacing of Δf (i.e., row A in FIG. 1). Assume the lowest frequency mode is at f0 and the highest frequency mode fnmax>2*f0 (i.e., greater than an octave in frequency). In general, f0 will not be an exact integer multiple of Δf, but f0 can be expressed as an integer multiple of Δf plus some offset frequency, δ. Therefore, f0=(p*Δf+δ). The frequencies of each of the laser modes can be defined as fn=f0+n*Δf=(p*Δf+δ)+n*Δf, where n and p are integers and δ is the offset frequency between the laser modes and a comb of frequencies defined by m*Δf, where m is an integer.
Many existing and proposed applications in the terahertz regime (e.g., 0.1-10 THz), such as astrophysics, high-resolution molecular spectroscopy, plasma diagnostics and manipulation of cold molecules, require very precise knowledge of the signal frequencies. Existing systems to generate absolute frequency combs use external non-linear elements to create harmonics or difference frequencies and then mix the created frequencies on a separate detector fast enough to see the offset between the laser modes and the comb separation. Recent developments in frequency combs have provided frequency measurement precision with one hertz (Hz) resolution over nearly all frequencies. However, systems to create absolute frequency combs are typically quite large and in the terahertz (THz) the comb is generated by inefficient down-conversion from optical frequency combs.